The Nexus 5 has no built in EFS backup partitions. EFS contains things like your MEID or IMEI... your data profiles.. radio settings. The EFS cannot be replaced... it is unique to your phone only...
Never flash another phones EFS to your phone.
LG NEXUS 5
SGS2(i9100)(retired to the draw)
If Someone helped you, but you can still see the Thanks button, you are doing it wrong.
Those who say it can't be done are often interrupted by people who are doing it.
Everyone is entitled to be stupid, but some abuse the privilege.